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Man Pages


Manual Reference Pages  -  PSLAQSY (l)

NAME

PSLAQSY - equilibrate a symmetric distributed matrix sub( A ) = A(IA:IA+N-1,JA:JA+N-1) using the scaling factors in the vectors SR and SC

CONTENTS

Synopsis
Purpose
Arguments
Parameters

SYNOPSIS

SUBROUTINE PSLAQSY( UPLO, N, A, IA, JA, DESCA, SR, SC, SCOND, AMAX, EQUED )
    CHARACTER EQUED, UPLO
    INTEGER IA, JA, N
    REAL AMAX, SCOND
    INTEGER DESCA( * )
    REAL A( * ), SC( * ), SR( * )

PURPOSE

PSLAQSY equilibrates a symmetric distributed matrix sub( A ) = A(IA:IA+N-1,JA:JA+N-1) using the scaling factors in the vectors SR and SC. Notes
=====

Each global data object is described by an associated description vector. This vector stores the information required to establish the mapping between an object element and its corresponding process and memory location.

Let A be a generic term for any 2D block cyclicly distributed array. Such a global array has an associated description vector DESCA. In the following comments, the character _ should be read as "of the global array".

NOTATION STORED IN EXPLANATION
--------------- -------------- -------------------------------------- DTYPE_A(global) DESCA( DTYPE_ )The descriptor type. In this case,
DTYPE_A = 1.
CTXT_A (global) DESCA( CTXT_ ) The BLACS context handle, indicating
the BLACS process grid A is distribu-
ted over. The context itself is glo-
bal, but the handle (the integer
value) may vary.
M_A (global) DESCA( M_ ) The number of rows in the global
array A.
N_A (global) DESCA( N_ ) The number of columns in the global
array A.
MB_A (global) DESCA( MB_ ) The blocking factor used to distribute
the rows of the array.
NB_A (global) DESCA( NB_ ) The blocking factor used to distribute
the columns of the array.
RSRC_A (global) DESCA( RSRC_ ) The process row over which the first
row of the array A is distributed. CSRC_A (global) DESCA( CSRC_ ) The process column over which the
first column of the array A is
distributed.
LLD_A (local) DESCA( LLD_ ) The leading dimension of the local
array. LLD_A >= MAX(1,LOCr(M_A)).

Let K be the number of rows or columns of a distributed matrix, and assume that its process grid has dimension p x q.
LOCr( K ) denotes the number of elements of K that a process would receive if K were distributed over the p processes of its process column.
Similarly, LOCc( K ) denotes the number of elements of K that a process would receive if K were distributed over the q processes of its process row.
The values of LOCr() and LOCc() may be determined via a call to the ScaLAPACK tool function, NUMROC:
LOCr( M ) = NUMROC( M, MB_A, MYROW, RSRC_A, NPROW ),
LOCc( N ) = NUMROC( N, NB_A, MYCOL, CSRC_A, NPCOL ). An upper bound for these quantities may be computed by:
LOCr( M ) <= ceil( ceil(M/MB_A)/NPROW )*MB_A
LOCc( N ) <= ceil( ceil(N/NB_A)/NPCOL )*NB_A

ARGUMENTS

UPLO (global input) CHARACTER
  Specifies whether the upper or lower triangular part of the symmetric distributed matrix sub( A ) is to be referenced:
= ’U’: Upper triangular
= ’L’: Lower triangular
N (global input) INTEGER
  The number of rows and columns to be operated on, i.e. the order of the distributed submatrix sub( A ). N >= 0.
A (input/output) REAL pointer into the local
  memory to an array of local dimension (LLD_A,LOCc(JA+N-1)). On entry, the local pieces of the distributed symmetric matrix sub( A ). If UPLO = ’U’, the leading N-by-N upper triangular part of sub( A ) contains the upper triangular part of the matrix, and the strictly lower triangular part of sub( A ) is not referenced. If UPLO = ’L’, the leading N-by-N lower triangular part of sub( A ) contains the lower triangular part of the matrix, and the strictly upper trian- gular part of sub( A ) is not referenced. On exit, if EQUED = ’Y’, the equilibrated matrix:
diag(SR(IA:IA+N-1)) * sub( A ) * diag(SC(JA:JA+N-1)).
IA (global input) INTEGER
  The row index in the global array A indicating the first row of sub( A ).
JA (global input) INTEGER
  The column index in the global array A indicating the first column of sub( A ).
DESCA (global and local input) INTEGER array of dimension DLEN_.
  The array descriptor for the distributed matrix A.
SR (local input) REAL array, dimension LOCr(M_A)
  The scale factors for A(IA:IA+M-1,JA:JA+N-1). SR is aligned with the distributed matrix A, and replicated across every process column. SR is tied to the distributed matrix A.
SC (local input) REAL array, dimension LOCc(N_A)
  The scale factors for sub( A ). SC is aligned with the dis- tributed matrix A, and replicated down every process row. SC is tied to the distributed matrix A.
SCOND (global input) REAL
  Ratio of the smallest SR(i) (respectively SC(j)) to the largest SR(i) (respectively SC(j)), with IA <= i <= IA+N-1 and JA <= j <= JA+N-1.
AMAX (global input) REAL
  Absolute value of the largest distributed submatrix entry.
EQUED (output) CHARACTER*1
  Specifies whether or not equilibration was done. = ’N’: No equilibration.
= ’Y’: Equilibration was done, i.e., sub( A ) has been re-
placed by:
diag(SR(IA:IA+N-1)) * sub( A ) * diag(SC(JA:JA+N-1)).

PARAMETERS

THRESH is a threshold value used to decide if scaling should be done based on the ratio of the scaling factors. If SCOND < THRESH, scaling is done.

LARGE and SMALL are threshold values used to decide if scaling should be done based on the absolute size of the largest matrix element. If AMAX > LARGE or AMAX < SMALL, scaling is done.

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ScaLAPACK version 1.7 PSLAQSY (l) 13 August 2001

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